1. Field of the Invention
The present invention relates to a method of detecting the characteristics of a piezoelectric element, and more particularly to a method of determining the equivalent serial resistance value of the piezoelectric element, an oscillation device that has a constitution suitable for such characteristic detection, and a vibrating gyroscope that comprises the oscillation device.
2. Description of the Related Art
Oscillation devices that employ a piezoelectric oscillator such as a quartz crystal or the like are used in a wide range of fields such as in various electronic devices such as a clock or reference-frequency frequency source used in the fields of electronics and communication. Examples of electronic devices that comprise such an oscillation device include an angular velocity sensor (gyroscope device), for example. The angular velocity sensor is used in the attitude control and navigation of moving bodies such as vehicles and detects the angular velocity of the moving body by detecting a Coriolis signal that corresponds with the angular velocity.
This oscillation device is miniaturized by integrating a piezoelectric oscillator and an LSI oscillation circuit by incorporating same in a package. Further, in order to increase the accuracy and reliability of the output frequency of the oscillation device, the characteristic of the piezoelectric oscillator which is the oscillating part must be tested and adjusted.
As a characteristic for judging whether or not a piezoelectric oscillator such as a quartz crystal is favorable, a value known as the crystal impedance (CI value) is known. This value is an equivalent constant that corresponds to the serial resistance value R when the piezoelectric element is represented by an equivalent circuit such as the one shown in FIG. 24.
Here, in the equivalent circuit in FIG. 24, the impedance Z is denoted by Z=1/(jωC0)+(1/(R+j(ωL−1ωC1))). The impedance Z when the oscillation circuit is made to oscillate at a frequency close to the serial resonance frequency (ω=1(LC1)1/2) is |ωC0|<<1 and, therefore, Z≈R. This serial resistance value R is called the crystal impedance (CI value), for example, and is used as the value representing the characteristic of the piezoelectric element.
Generally speaking, in order to determine the serial resistance value of a piezoelectric element that an oscillation device comprises, a constitution in which electrodes that are electrically connected to the piezoelectric element are provided outside the oscillation device is employed. FIG. 25 is an example for connecting a conventional constitution that determines a serial resistance value of the piezoelectric element. FIG. 25A shows an example of a single oscillation device and FIG. 25B shows an example of a gyroscope device that comprises an oscillation device.
In the case of a single oscillation device, as shown in FIG. 25A, an external terminal (Xin terminal) 120 and an external terminal (Xout terminal) 121 are provided in an oscillation circuit 130 that constitutes an oscillation device 101, and an LCR meter 140 is connected to the external terminals (Xin terminals) 120 and 121. The respective equivalent constants of the piezoelectric elements 2 are measured by the LCR meter 140. The oscillation circuit 130 comprises an output terminal 131.
In addition, even in cases where the oscillation device 101 is built into the gyroscope device 110, as shown in FIG. 25B, for the oscillation device 101 that is built into the gyroscope device 110, the LCR meter 140 is connected to the external terminals 120 and 121 as mentioned earlier and the respective equivalent constants of a piezoelectric element 2 are measured by means of the LCR meter 140. A detection circuit 160 comprises an output terminal 161. The oscillation circuit 130 is connected with the oscillation circuit 160 comprising the output terminal 131 through a phase circuit 150.
FIG. 26 shows the relationship between the serial resistance value (CI value) that is actually measured by the LCR meter and the drive voltage (Xout) of the oscillation circuit. Further, here, the drive voltage of the oscillation circuit is for a case where the oscillation circuit performs constant current control. Therefore, it can be seen from FIG. 26 that the voltage characteristic of the output can be judged from the serial resistance value (CI value) and, if the serial resistance value (CI value) is in a predetermined range, the output voltage of the oscillation circuit can be judged to be in the predetermined range.
In addition, Japanese Application Laid Open No. 2001-102870 discloses a constitution that is suitable for testing a crystal oscillator comprising a quartz crystal. According to Japanese Application Laid Open No. 2001-102870, tests on the quartz crystal include a drive level characteristic (DLD characteristic) test that measures the oscillation frequency and the fluctuations corresponding to the quartz crystal loss when the drive level applied to the quartz crystal is changed and the problem that DLD characteristic testing is problematic due to the structure in an oscillator that incorporates a quartz crystal and an oscillation circuit in a package is mentioned.
Methods of solving the above problem include a constitution that provides electrodes that are electrically connected to the electrodes of the quartz crystal outside the oscillator. Japanese Application Laid Open No. 2001-102870 points out the fact that, due to the electrode constitution, there are problems such as the fact that the routing of the wiring is long, there is an invasion of noise into the quartz crystal terminal via the external terminals and, the oscillation operation is unstable and, as a constitution for solving these problems, illustrates a constitution in which, in addition to the bottom face of the package being provided with terminals such as a power source input terminal, a ground terminal, a frequency output terminal, and a control terminal, the bottom face or sides of the package comprise a piezoelectric oscillator testing terminal, the testing terminal being electrically activated when the piezoelectric oscillator is being tested and being electrically de-activated during the oscillation operation.
As mentioned earlier, there is the problem that, in order to determine the characteristics of the piezoelectric element that the oscillation device comprises and, in particular, the equivalent serial resistance value of the piezoelectric element, a measuring device such as an LCR meter must be connected to the outside and a terminal for connecting the measuring device must be provided.
In addition, the piezoelectric element and oscillation circuit that constitute the oscillation device are normally built into a package and an external terminal that is electrically connected to the circuit inside the package must be provided on the outer surface of the package in order to test and adjust the circuit contained in the package. The terminal for performing the testing or adjusting is used in a state where the package alone exists, prior to attaching same to an external device such as a mounting substrate. However, when the package is mounted on an external device after the testing and adjustment are complete, the terminal is then unnecessary because the terminal is not being used. There is also the problem that a potential route for the invasion of external noise is created because the terminal is connected to a circuit inside the package.
Therefore, the ability to measure the characteristic of the piezoelectric element that the oscillation device comprises and, in particular, the equivalent serial resistance value of the piezoelectric element without connecting a measuring device such as an LCR meter to the outside is sought as well as the ability to dispense with the terminal for such measurement.
A terminal for testing and adjusting circuits built into the package that combines a function to establish an electrical connection with a circuit inside the package prior to mounting with a function for preventing the invasion of noise into the circuit in the package in cases where the package is attached to an external device is also sought.
In the aforementioned Japanese Application Laid Open No. 2001-102870, although the testing terminal is electrically de-activated during the oscillation operation, the testing terminal is in an OFF state in an analog switch in the circuit and at an unstable potential in a floating state with respect to the package. It is therefore difficult to completely prevent the invasion of noise.